The chemical recycling of polyethylene terephthalate, PET, post consumption, consists of a set of procedures to promote its depolymerization and regenerate its raw materials: terephthalic acid and ethylene glycol. Part of the procedures is mechanical, such as the collection of PET pieces, such as beverage bottles and discarded artoc;es in general, and the transportation, compaction, baling, comminution, elimination of undesired polymers, washing, drying and other complementary processes performed to aggregate to produce so called “solid residue”.
Another part of these procedures includes the depolymerization of PET into terephthalic acid and ethylene glycol, and the subsequent purification of these products. Each one of the procedures utilized herein has its appropriate equipment for its performance.
Distinct processes for the production of terephthalic acid are in the state of the art. There are also the process for producing the dimethyl terephthalate, instead of terephthalic acid, due to the difficulty of purification of the terephthalic acid.
In the polymerization for the production of PET, dimethyl terephthalate provides worst polymerization conditions than terephthalic acid, such as for example a lower polymerization rate, a higher consumption of ethylene glycol and formation of residual methanol.
Therefore, the manufacturing processes of said diester lost preference and the route more employed for the manufacturing of terephthalic acid consists in the catalytic oxidation in liquid phase of p-xylene to terephthalic acid.
The processes are based on the formation of a dimethyl ester of terephthalic acid, where a catalytic oxidation is processed in liquid phase of p-xylene 1 and methyl p-methyltoluate 5, with air oxygen 2, producing p-toluic acid 3 and methyl mono ester of terephthalic acid 6 and water 4 as co-product.
This mixture is esterified with methanol 7, producing the methyl p-methyltoluate 5 and dimethyl terephthalate 8, as presented in FIG. 1.
After the esters are separated by distillation, the dimethyl terephthalate 8 can be hydrolyzed into terephthalate acid 9.
The p-carboxyaldehyde 10, formed as an intermediate in the p-xylene 1 oxidation with oxygen 2, is also esterified with methanol 7 into methyl p-carboxytoluate 11 and this ester, by its turn, is hydrolyzed with water 4 regenerating the p-carboxyaldehyde 10 as is also presented in FIG. 1.
The direct oxidation into terephthalic acid consists in a chemical process of structure change of the raw material, p-xylene, which is an aromatic hydrocarbon, to the structure of a dicarboxylacid, in this case terephthalic acid.
This processing is performed in continuous chemical reactors, where the following reactions presented in FIG. 2 occur, where the p-xylene is oxidized by air oxygen 2, in the presence of acetic acid as solvent and cobalt acetate, sodium bromide, carbon tetrabromide as catalyst/co-catalyst, at 175-230° C. and 15-35 bar, forming the p-toluic acid 3 and this, by its turn, is oxidized to terephthalic acid 4.
Actually, the main reactions are a simplification of the complete mechanism presented in FIG. 3, where the intermediate steps of p-xylene 1 oxidation into p-methylbenzyl hydroperoxide 12, from this to p-tolualdehyde 13, then to p-toluic acid 3, then to p-carboxyaldehyde 10 and finally to terephthalic acid 8. As the reactions show, it is a process occurring in successive oxidation steps. If the intermediate oxidation reactions are not taken to its own complementation, at the end of the process, intermediate products such as process undesired by-products will remain. The control over these serial reactions will determine the degree of contamination of the terephthalic acid produced and define its impurities.
The intermediate substances accompanying the terephthalic acid cause problems to the polymerization process in PET manufacturing, for example, the p-methylbenzoic acid 3 delays the polymerization and leads to the obtainment of a low-molecular weight polymer. Another example is the p-carboxyaldehyde 10 that causes the coloration in the terephthalic acid.
The main purification step of the raw terephthalic acid is its hydrogenation, in aqueous suspension and in the presence of a palladium coal-supported catalyst, at 250° C., when the p-carboxyaldehyde 10 is reduced to toluic acid 3. A subsequent purification is the terephthalic acid 8 crystallization. And the final purification consists in a sublimation of the re-crystallized terephthalic acid. Only with this purification sequence is it possible to reach the purity degree required for terephthalic acid to be appropriate to the production of PET. All this work is due to the formation of p-carboxyaldehyde, which is admitted, at most, in a level of 25-50 ppm in polymerization grade terephthalic acid.
The sublimation is a product based on the steam pressure value of the solid terephthalic acid and it is a single, slow, operation, requiring large volume equipment due to the low mass and heat exchange rate during sublimation. There is also an aggravating point: according to the intensity of heating at sublimation, the terephthalic acid formed transforms into terephthalic anhydride.
In order to avoid that this new impurity follows the product intended to polymerization, the sublimated terephthalic acid is treated with water steam and, later, subjected to drying.
According to the state of the art, a manufacturing route of ethylene glycol 16, main diol used in the PET manufacturing, is the water 4 hydrolysis of the ethylene oxide 15, manufactured from the ethylene 14 by catalytic oxidation with air oxygen 4, as summarized in FIG. 4.
According to the state of the technique, the PET recycling can be classified into two large universes: mechanical recycling (where the PET chemical structure is not altered) and chemical recycling (where the original PET structure goes through a molecular change).
The mechanical recycling presents successive physical operations viewing to aggregate value to the solid residues constituted by PET, conducting the different articles manufactured in PET to the shape of flakes and granulate. The PET flakes are particles with millimeter dimensions of PET, obtained by communition of PET residues, and that might be marketed, within the recycling chain, for the production of granulates. The granulates are PET particles obtained by the melting of flakes and, subsequent melted material granulation, and constitute the basic final product of the mechanical recycling line of post consumption PET.
The steps of the transformation of PET articles into flakes and/or granulates comprise the following sequence of operations: a) collection of post consumption PET articles, i.e., selective collection of PET wastes from urban garbage; b) classification, done in mats that transport the acquired material while operators select the elements that are not PET and remove them from the mat; c) milling and washing, which is done in humid mills, where PET is comminuted; d) rinsing, done in two transporting threads, where the washing water is separated; d) separation and decontamination, which is done in a tank an endless thread to remove materials different from PET; f) pre-drying, which is made in a vertical centrifuge, where the water accompanying the PET flakes is separated; g) drying and dust elimination, which is done in a continuous hot-air electric drier, where the flake is dried and the dust formed is dragged in the air; h) particles classification, which is done in a vibrating sieve, where the PET flakes are separated according to their granulometry; i) bagging, which is done by a bagger.
A later treatment that aggregates value to recycled PET is its granulation by controlled heating, when the flakes are transformed into granules.
The chemical recycling can be understood at two levels: “recondensation” level (where the PET granulate is treated in order to increase its mean molecular weight) and the depolymerization level (where the PET molecule is totally destroyed, yielding terephthalic acid and ethylene glycol).
The recondensation views to correct the mean molecular weight of the recycled PET. During the conformation processes of the virgin PET and mechanical recycling processes of post consumption PET, the polymer molecules suffer heating and mechanical stresses causing a certain degree of breaking of these macromolecules, resulting in a decrease of mean molecular weight.
The recondensation consists in a chemical process where the recycled polymer is subjected to high temperatures and high vacuum, in the presence of catalysts, forcing the broken molecules to react among themselves and increase the mean molecular weight (U.S. Pat. No. 6,436,322, U.S. Pat. No. 4,657,988, CA 1277081). The inconveniency of this process is that the molecular weight distribution profile is not remade, but only the mean molecular weight is increased to the levels of the original polymers.
The depolymerization is based in the hydrolysis reaction, which is a typical reaction of esters, PET is a polyester, i.e., a macromolecule constituted by the repetition of interlinked monomers by the chemical bond between the molecular structures of the terephthalic acid and ethylene glycol.
FIG. 5 shows the formation of the ester binding in the PET 17 polymerization. This chemical bond between an acid (in this case, terephthalic acid) and an alcohol (in this case, ethylene glycol) is called ester binding and the product constitutes an ester (in this case, PET).
The esters are susceptible to a series of reactions, among them with water 4, organic acids 18, alkalis such as soda 20 and alcohols 22 reactions. In these four reactions, the ester binding is broken and in the case of PET 17, the following are respectively formed: terephthalic acid 10 and ethylene glycol 16—this reaction is called hydrolysis; terephthalic acid 10 and ester of the acid employed with ethylene glycol 19—this reaction is called acidolysis; sodium terephthalate 21 and ethylene glycol 16—this reaction is called saponification; terephthalate 23 of alcohol 22 used and ethylene glycol 16—this reaction is called alcoholysis. In FIG. 6 a table is presented showing these reactions.
From this point of view of the chemical reactions, the acidolysis, saponification and alcoholysis are particular cases of the hydrolysis reaction. The hydrolysis itself would be the ester reaction with water, both without catalysis or with acid catalysis (acid hydrolysis) or alkaline (alkaline) hydrolysis). Note that the difference between acidolysis or saponification and acid or alkaline hydrolysis is the amount of acid or alkali used. In the first two cases, the amounts are the stoichiometrically required for a complete reaction of the ester. In the second case, the acid and alkali enter only in catalytic proportions (very small compared to the amount of ester) and are intended to promote the reaction mechanisms faster than the ones of a hydrolysis with no catalysis.
Some patented processes for the performance of this hydrolysis reaction without catalyst (U.S. Pat. No. 3,120,561), acid hydrolysis (U.S. Pat. No. 3,355,175), alkaline hydrolysis and saponification (WO 95/10499, U.S. Pat. No. 6,031,128, U.S. Pat. No. 4,193,896), acidolysis (WO 03033581, U.S. Pat. No. 5,948,934) and alcoholysis (U.S. Pat. No. 5,559,159) are in the state of the art. Such processes, based on the hydrolysis process, according to the state of the art, generically consist in the PET comminution, in the polymer hydrolysis and in the purification of the terephthalic acid obtained. This purification consists in a precipitation of the terephthalic acid and subsequent purification by crystallization. The terephthalic acid produced by this way still needs the sublimation purification in order to be classified with a polymerization grade, due to the presence of low molecular weight oligomers formed as intermediates during the hydrolysis reactions.